The present invention relates to a bar code reader of the type which scans a light beam over an area (i.e. raster scan) rather than a line. In particular, the reader includes a first oscillating mirror for scanning the light parallel to a first line (e.g. vertical line) at a first frequency, and a second oscillating mirror for scanning light from the first mirror parallel to a second line (e.g. horizontal line perpendicular to the vertical line) at a second frequency greater than the first frequency.
Many applications such as industrial applications require bar code readers to search for bar codes passing the scanner with the bars and associate spaces of the bar codes having substantially vertical orientations (i.e. a picket fence orientation). To read these bar codes, the reader needs to scan an area instead of a single line. Thus, the reader must be capable of scanning in a raster pattern to cover the area. To provide the ability to scan a raster pattern, typical readers include either a multi-mirror polygon or a moving mirror (post-raster mirror) for scanning light received from a high speed oscillating mirror.
The primary problem with using a multi-mirror polygon to generate a raster pattern is that the polygon needs to have a substantial number of faces to generate a sufficiently wide (high) raster pattern (i.e. scan area). Typically, each mirror of the polygon needs to be one (1) inch square. Furthermore, a polygon which can generate only a six (6) degrees scan angle requires 20 mirrors. Accordingly, the polygon would have a diameter of almost twelve (12) inches, have a substantial weight (e.g. three (3) pounds), and have a substantial cost. Furthermore, even if these problems can be dealt with, a six (6) degree scan angle is no longer satisfactory for modern bar code readers. Rather, modern bar code readers require scan angles in the range of thirty (30) degrees. Assuming a very wide, potentially unacceptable, scan line separation of 0.5 degrees, a polygon capable of producing a thirty (30) degree scan angle would require 60 mirrors and have a diameter of three (3) feet. This size is unacceptable for use in modern scanners. Thus, multi-mirror polygons are not practical for use to produce raster patterns having scan angles greater than only a few degrees.
One problem with post-raster mirrors is the requirement that such mirrors be relatively large. Without a relatively large post-raster mirror, a sufficient amount of light reflected from a scanned code can not be collected for reflection back to the photodetector of the reader. Reflection of an insufficient amount of light from a scanned code to the photodetector results in inaccurate code reading and/or reduced code reading capability. Furthermore, as the size of the scanned area increases, the size of the post-raster mirror must also increase.
Increasing the post-raster mirror size introduces a number of problems including manufacturing and mechanical support problems. Due to the location of the post-raster mirror in the optical path, substantially the whole surface of the mirror must be manufactured with a surface which is flat within 1/4 of a wavelength. Thus, as the mirror size increases, the thickness, inertia and cost of the mirror increase disproportionately. Furthermore, as the inertia (weight) of the mirror increases, the ability to mount the mirror directly to the shaft of a galvo or other type of motor is lost. Thus, the mirror must be supported with a separate shaft and bearings, and must be driven from motor or other appropriate device through a drive linkage. As a result, the inertia of the mirror and mirror drive increase and the energy required to oscillate the mirror increases.
In view of the problems associated with producing scan areas with multi-mirror polygons and post-raster mirrors, it would be desirable to provide an mirror oscillating assembly which cooperates with a raster mirror to increase scan area, reduce mirror cost and size, eliminate the need for a separate shaft and bearing support, and increase the amount of scanning light reflected onto the photodetector from the scanned code.